1. Field of the Invention
This invention relates to forehearth systems for conditioning molten glass from a glass melter and rendering it suitable for subsequent processing, such as forming it into a desired shape. More particularly, this invention relates to burner systems suitable for use in forehearth systems, and in particular, forehearth systems employing air-fuel and oxy-fuel forehearth burner block geometries.
2. Description of Prior Art
Glass melting furnaces require extremely high process temperatures for glass production. As a result, furnace flame temperatures and, thus, NO.sub.x generation, are high. As a result of the 1990 Clean Air Act, many regional municipalities now impose NO.sub.x emission limits on glass furnaces.
Throughout all segments of the glass industry, glass manufacturers have been adopting oxygen-fuel technologies to meet the increasingly stringent NO.sub.x emission limits. Substituting oxygen for air in the combustion process reduces NO.sub.x emissions and yields a more fuel efficient process, resulting in reduced CO.sub.2 output. In addition to improved furnace emissions and fuel efficiency, oxy-fuel furnaces provide for more stable furnace operation and higher quality glass. At the present time, glass manufacturers are investigating the feasibility of converting air-fuel fired forehearths to oxy-fuel fired forehearths.
The forehearth section of a glass manufacturing operation is disposed between the glass melting furnace in which the raw materials for making glass are melted and the processing section in which the molten glass is processed into a desired form or shape. The forehearth system is designed to receive molten glass from the glass melting furnace and convey it to the glass processing operation, conditioning the molten glass during conveyance between the glass melter and the glass processing operation, thereby rendering it suitable for processing. In particular, the forehearth is designed to heat or cool the glass to the temperature required for processing. For some glass products, the temperature of the molten glass must be maintained within 1.degree. or 2.degree. F. at the inlet to the glass forming process. Conventional forehearths obtain temperature control through a series of small air-gas premix burners placed longitudinally on either side of the forehearth duct. Spacing between forehearth burners is typically between six and eighteen inches. A single forehearth can contain several hundred forehearth burners and a single glass furnace may have one or multiple forehearths.
Typical forehearth systems comprise a cooling section which receives molten glass from the melter portion of the furnace, and a front conditioning section which receives molten glass from the cooling section. The conditioning section lies between the cooling section and the glass processing section. The cooling and the conditioning sections are provided with independently controlled firing systems.
The cooling section of the forehearth system receives molten glass from the melter and cools or heats it to the proper average temperature required for the type of glass being made, such as containers made by a forming machine or fibers stretched by various attenuation devices. When the desired glass temperature cannot be obtained by radiation alone while maintaining properly set flames above the molten glass, additional cooling air is introduced into the cooling section of the forehearth above the molten glass.
From the cooling section, the glass flows into the conditioning section of the forehearth in which the temperature of the glass is equalized only by heating, using burners disposed within the walls of the forehearth, and not by cooling. The temperature in the conditioning section is controlled independently of the temperature in the cooling section. The conditioning section is intended only to hold and equalize the temperature and, thus, the viscosity of the glass.
Traditional firing systems for heating the glass in the conditioning section of a forehearth system are of a combustion premix design in which the fuel, for example natural gas, and combustion air are premixed together before they are introduced into the burner. See, for example, U.S. Pat. No. 5,169,424 which generally teaches a forehearth structure for a glass melting furnace including gas burners for providing heat to the molten glass flowing through the forehearth. See also U.S. Pat. No. 4,662,927 which teaches a forehearth system having fuel-air burner nozzles which provide a flame for heating the space above the flowing molten glass and U.S. Pat. No. 4,708,728 which teaches a premixed fuel-air burner for heating the forehearth of a glass melter, the burner having a capillary tube disposed coaxially therein and extending beyond the end of the burner for feeding oxygen into the fuel-air premixture.
Numerous problems exist for traditional forehearth firing systems which employ premixed air-fuel burners for heating the flowing gas, including poor fuel efficiency, little or no flame luminosity, very limited turn down ratio, a high volume of combustion gases and associated emissions within and outside of the glass plant, a generally high noise level due to the air-gas combustion system and, finally, the inability to provide precise temperature control of the glass, as small as 1.degree. or 2.degree. F. due to the significant variations in atmospheric air used by air-gas firing systems.
These problems can be addressed by the use of oxygen-utilizing combustion systems. The use of oxy-fuel fired forehearths results in reduced emissions and fuel consumption as well as better temperature control for improved glass quality. For example, a 100% oxy-fuel combustion system can reduce fuel consumption by about 60% compared to air-fuel combustion without any heat recovery. U.S. Pat. No. 5,147,438 teaches an auxiliary oxy-fuel burner for glass melting having a central fuel nozzle disclosed concentrically within an oxygen nozzle; U.S. Pat. No. 4,531,960 teaches a process for making glass using air-fuel and oxygen-fuel burners where the flame produced by the oxygen-fuel burners is surrounded by a current of auxiliary gas, such as air or nitrogen, introduced through an annular space surrounding the burner; and U.S. Pat. No. 5,092,760 teaches an oxy-liquid fuel burner where oxygen or carbon dioxide are used as an atomizing fluid for the liquid fuel. U.S. Pat. No. 5,500,030 teaches an oxy-gas forehearth burner which produces a luminous pencil-like shaped flame. This type of flame, when fired through an existing air-gas forehearth burner block recirculates particulates and combustion gases up through the forehearth burner block. This recirculation accelerates burner tip corrosion and plugging, which leads to premature failure of the burner. In addition, because the gas and oxygen flow streams are parallel to each other upon exit from the burner, mixing is delayed, resulting in a long flame which can impinge on the opposing wall of a narrow forehearth. For optimal performance and burner life, the burner taught by this patent should be fired through a specially designed burner block for oxy-gas firing. Thus, retrofitting of existing air-gas fired forehearths with oxy-gas forehearth burners of the type taught by the '030 patent also requires replacement of the burner block. Thus, due to the multitude of existing air-gas forehearth burners in a forehearth system, it is advantageous to utilize oxy-gas forehearth burners which are retrofittable to existing air-gas forehearth burner block geometries, thereby obviating the need for replacement of existing air-gas forehearth burner blocks with specially designed burner blocks for oxy-gas firing.